Head-Mounted Displays (HMDs) are a class of image display devices that can be used to display images from television, digital versatile discs, computer applications, game consoles, or other similar applications. A HMD can be monocular (a single image viewed by one eye), biocular (a single image viewed by both eyes), or binocular (a different image viewed by each eye). Further, the image projected to the eye(s) may be viewed by the user as complete, or as superimposed on the user's view of the outside world. For most HMDs, designs must account for parameters such as image resolution, the distance of the virtual image from the eye, the size of the virtual image (or the angle of the virtual image), the distortions of the virtual image, the distance between the left and the right pupil of the user (inter pupillar distance (IPD)), diopter correction, loss of light from image splitting and transmission, power consumption, weight, and price. Ideally, a single HMD would account for these parameters over a variety of users and be able to display an image regardless of whether it was a stereoscopic binocular image or a simple monoscopic binocular image.
If the resolution of a picture on the HMD's internal display is 800×600 pixels, an acceptable size for the virtual image produced by the HMD's optics is a virtual image diameter of approximately 1.5 m (52″–56″) at 2 m distance which corresponds to approximately a 36° angle of view. To properly conform to the human head and eyes, the IPD should be variable between 45 mm and 75 mm. In order to compensate for near- and farsightedness, at least a ±3 diopter correction is necessary.
The use of only one microdisplay in the HMD (instead of using one for each eye) drastically reduces the price of the device. Typically, an arrangement for such a unit positions a microdisplay between the user's eyes. The image produced is then split, enlarged, and separately transmitted to each eye. There are numerous designs known in the art for beam splitting in single display HMDs with a center mounted display, but none provide a solution that is cheap, light weight, small in size, and capable of displaying all varieties of images.
Many applications for head mounted systems require that different information be transmitted to a right-eye of a user than is transmitted to a left-eye of a user. For example, to impart to a user a three-dimensional image requires that each eye of the user view a different prospective of the same image. In other applications, such as a system for projecting data on a user's view (sometimes referred to as a “heads-up display”), completely unrelated data may need to be transmitted to each eye.